1. Technical Field
This invention relates to a phosphor thin film for use in electroluminescent (EL) devices, a method of preparing the same and an EL panel using the same.
2. Background Art
In the recent years, active research works have been made on thin-film EL devices as small-size or large-size, lightweight flat display panels. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a double insulation structure using thin-film insulating layers 4A and 4B as shown in FIG. 5. In FIG. 5, a predetermined pattern of lower electrodes 3A is formed on a glass substrate 2, and a dielectric thin film is formed as a lower insulating layer 4A on the lower electrode-bearing substrate 2. On the lower insulating layer 4A, a light-emitting layer 5 in the form of a phosphor thin film and an upper insulating layer 4B are successively formed. On the upper insulating layer 4B, a predetermined pattern of upper electrodes 3B is formed so as to construct a matrix with the lower electrodes 3A. As a general rule, the phosphor thin film is annealed at temperatures below the strain point of the glass substrate 2 in order to enhance luminance.
More recently proposed was a structure using a ceramic substrate as the substrate 2 and a thick-film dielectric layer as the lower insulating layer 4A. Another device structure was proposed in which a high permittivity BaTiO3 thin plate is used as the substrate and an electrode is formed on the back of the substrate so that the thin plate serves as an insulating layer and substrate. Since ceramics such as alumina and BaTiO3 are used as the substrate, these structures permit the phosphor thin film to be annealed at high temperatures for providing an increased luminance. Also, since a thick film or thin plate dielectric layer is used as the insulating layer, these structures are resistant to insulation breakdown as compared with EL devices using a thin film as the insulating layer. Advantageously, more reliable devices can be manufactured. Then a structure of sandwiching a phosphor thin film like the double insulation structure is not necessarily needed. The insulating layer may be a single thick film or thin plate dielectric layer only on one side.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primary colors of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescence center), SrGa2S4:Ce and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primaries of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. Referring to the blue color among others, a relatively high luminance is achieved using SrS:Ce. However, as the blue color for full color display, its chromaticity is shifted toward green. There is a desire to have a better blue light-emitting layer.
To solve these problems, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Shingaku Technical Report, EID 98–113, pp. 19–24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291–1292. These thiogallate base phosphors are satisfactory in color purity, but suffer from difficulty to form a thin film of uniform composition because of the multi-component composition. It is believed that thin films of quality are not obtainable because of poor crystallinity resulting from inconvenient composition control, formation of defects resulting from sulfur removal, and admittance of impurities; and these factors lead to a failure to increase the luminance. Also the thiogallate and thioaluminate base phosphors require high processing temperatures for thin film formation, that is, high annealing temperatures of 750 to 900° C. after film deposition. This gives rise to several problems. The substrate material is limited because the substrate must be noticeably heat resistant; diffusion of elements occurs from the substrate or adjacent layer (insulating layer or the like) to the phosphor thin film; interlaminar flatness is compromised; interlaminar separation occurs during high-temperature annealing; pixels are destroyed by surface diffusion during high-temperature annealing; and a thermal guard must be provided on the annealing apparatus for high-temperature annealing, which adds to the expense.
In order to develop practical full color EL panels, phosphor materials capable of establishing blue, green and red phosphors in a consistent manner and at a low cost are necessary. The processing temperature at which phosphor thin films are processed differs with individual materials as mentioned above. This means that in a full color panel requiring three colors RGB to be disposed within it, the conditions under which the respective phosphor thin films are formed for providing the desired emission characteristics differ among them, rendering panel manufacture difficult. In particular, since the thioaluminate and thiogallate base materials require high processing temperatures as mentioned above, it is desired to lower the processing temperature. More particularly, it is desired that red, green and blue phosphor thin-film materials capable of high luminance emission be simultaneously formed and annealed at low temperatures.